It is well known that certain types of fluorescent materials, when exposed to radiation—whether X rays, alpha rays, beta rays, gamma rays, electron beams or ultraviolet light—store a certain portion of the energy with which they are irradiated so that, when irradiated with visible or other excitation light, these fluorescent materials undergo accelerated phosphorescence. Such materials are called photostimulable phosphors.
In Japanese Laid-Open Patent Application Nos. 55-12429, 56-11395 and the like, radiation image information recording and reproduction systems that utilize this photostimulable phosphors have been proposed. According to such systems, radiation image information from a human body or other object exposed to radiation is first recorded on a photostimulable phosphor sheet. The photostimulable phosphor sheet is then exposed to a laser beam or other excitation light, causing the photostimulable phosphor sheet to undergo accelerated phosphorescence. The accelerated phosphorescence is then read photoelectrically to acquire image signals, which are then used to make visible a radiation image of the irradiated object by displaying it on a photosensitive recording medium, CRT or the like.
Recently, apparatuses that use semiconductor sensors similarly to take X-ray images have been developed. Systems such as these have the practical advantage of being able to record images over a considerably wider range of radiation exposures than radiation photo systems using silver chloride film can do. In other words, such systems can provide visible radiation images unaffected by fluctuations in radiation exposure amounts, by acquiring X rays over an extremely broad dynamic range using photoelectric conversion means, converting the acquired X rays into electrical signals, and using the electrical signals to output a visible image to a photosensitive recording medium, CRT or other display apparatus.
The electrical signals acquired as described above can be converted into digital information, and the digital information so obtained can be stored in a storage medium such as a memory. This type of digital information can then be supplied to an information processing apparatus for digital image processing, in order to provide various types of diagnostic support.
However, X-ray images contain a large amount of information, so storing and transmitting such images involves very large volumes of data. Advanced encoding is used to reduce the tremendous volumes of data involves in storing and receiving such X-ray images, by eliminating the image redundancy or by altering the image content so as to degrade the image by an amount still not easily discernible to the naked eye.
Thus, for example, in the Joint Photographic Experts Group (JPEG) standard recommended by the International Standard Organization (ISO) and International Telecommunication Union (ITU) as the international standard coding method for still pictures, Differential Pulse Code Modulation (DPCM) is used for reversible compression and Discrete Cosine Transform (DCT) is used for non-reversible compression. A detailed explanation of JPEG is provided in ITU-T Recommendation T.81, ISO/IEC (International Electrotechnical Commission) 10918-1 and so will be omitted here.
Additionally, much recent research has concentrated on compression methods using Discrete Wavelet Transform (DWT). The advantage of DWT is that the blocking artifacts seen with DCT do not appear.
On the other hand, it is possible to improve the efficiency of compression when compressing an X-ray image by determining an area of interest within the image and reducing the data compression ratio for that area so as to provide superior picture quality for it. Also, when performing lossless coding as well, it is possible to priority code an area of interest (hereinafter sometimes referred to as AOI) and then priority decode that same area when reading out the image. However, the determination of an AOI is not an easy choice to make, involving as it does a medical judgment.
With these considerations in mind, the applicant has proposed a method (and apparatus) that, when an image is compressed, analyzes an input image to extract the X-ray radiation field area, then further extracts an X-ray pass-through region from the extracted radiation field area, sets the area of interest as that part of the radiation field area other than the pass-through region, level-shifts and codes the image corresponding to the area of interest so as to priority code the area of interest. The problem with this method is that the pass-through region within the carved-out image (that is, the radiation field area) constitutes approximately 20 percent of the field, so in order to improve the data compression ratio the AOI must be narrowed further.
Additionally, there are many instances in which it is useful to set the area of interest when displaying the contents of the compressed image file. For example, displaying one or more images composed of 2,000×2,000 pixels on a monitor of 1,000×1,000 pixels involves a choice between either displaying only a portion of one image or displaying reduced image(s). Of course, it is possible to display the entire unreduced image and simply scroll down the screen using a mouse or a track ball, but such an expedient will not achieve the objective of simultaneously displaying a plurality of images. It is possible to display only the AOI if the AOI is set for the images when the images are being filed, but if there is no AOI, or if the AOI has been set but overflows its allotted space, in either case it is necessary to use a reduced display.
A reduced display creates its own problems, insofar as such display has poor spatial reduction capabilities, which can make detailed inspection of an image impossible. Techniques have been proposed to compensate for this defect, for example, use of a normalized rectangle or circular magnifying lens that an operator (a physician) moves across the top of the CRT so as to partially display the original subject. However, the problem remains as to whether or not it is possible to select appropriately an area to enlarge, given that such display has poor spatial resolution capabilities.
It is also sometimes the case that X-ray images are encoded before storage so as to reduce the volume of data involved, but it is preferable that such encoded images be displayed promptly, without delay. The ability to display such encoded images promptly makes it possible also to priority display an area useful to the diagnosis from the encoded image data.